The invention generally relates to electronically controlled motors and, in particular, to a controller providing a desired operating area for a motor driving a fan for inducing a draft in an exhaust.
In a conventional furnace, natural convection currents carry away exhaust gases from the furnace's combustion chamber. However, the resultant loss of heat energy in the exhaust gases decreases the overall efficiency of the furnace. To improve furnace efficiency, heat exchangers are used to extract additional heat from the exhaust gases before they are vented to the atmosphere. Extracting heat from the exhaust gases, however, reduces the natural convection currents which would otherwise carry the gases away. One solution has been to use a draft inducing fan to exhaust the exhaust gases into the atmosphere.
Prior designs for a draft inducer control system controlling the exhaust of a furnace include a pressure transducer located in the vent adjacent the combustion chamber, a speed sensor for the motor and a torque sensor for the motor. These components vary the induced draft as a function of the sensed pressure in the combustion chamber. Such a control system, for example, is disclosed in commonly assigned U.S. Pat. No. 5,075,608, the entire disclosure of which is incorporated herein by reference.
Commonly assigned application Ser. No. 08/025,371, now U.S. Pat. No. 5,418,438, the entire disclosure of which is incorporated herein by reference, provides improvements in draft inducer systems which beneficially increase the efficiency of a furnace. For example, improvements which allow the speed/torque of the motor and the magnitude of the induced draft to be preselected for given operating conditions of the furnace increases efficiency by more closely matching the induced draft to the requirements of the furnace. Other improvements which allow this preselection to occur quickly and economically during manufacture or installation of the furnace and which provide greater versatility of the draft inducer control system in responding to various control signal conditions are also available.
While such systems represent improvements, further improvements in draft inducer control systems, draft inducer apparatus, and methods of control and operation are needed to beneficially provide an area of desired motor operation to minimize overheating in such systems. For example, a motor driving a draft inducer generally has the ability to provide relatively high torque at variable speeds. However, operation of the motor at relatively low speeds when torque is excessive, or under abnormal operating conditions, can result in increased operating temperatures. If the motor exceeds allowable operating temperatures, its electronic control can fail. While conventional motor systems provide various controls affecting motor speed, current and torque, such systems fail to provide adequate protection against the generation of excessive heat. Typically, motor systems limit the production of torque by their motors to a maximum torque independently of speed. Also, such systems may include an overcurrent or overspeed protection system which operates by preventing current flow to the motor or by disconnecting voltage from the motor. Although presently available systems provide for maximum torque control and control against overcurrent and overspeed conditions, there is a need for a motor system which maintains motor operation in a desired area with respect to motor speed/torque and which provides a maximum current limit as a function of motor speed.